This invention relates to ABS-type polymers and more particularly to an improved process for preparing ABS-type polymers having excellent physical properties.
Over the past few years acrylonitrile-butadiene-styrene polymer, more commonly known as ABS polymers, have taken over a large share of the market formerly held by other impact polymeric materials such as impact polystyrene. The constantly increasing demand for these polymers is due to the excellent mechanical and chemical properties that they possess; for instance ABS plastics have greater toughness and impact strength than impact polystyrenes containing the same amounts of rubber and they also possess excellent chemical solvent resistance.
In spite of the excellent mechanical and chemical properties of ABS polymers their growth in the plastics market has been somewhat hindered due to disadvantages associated with the method of commercial production of ABS polymers. Today, by far the greatest amount of ABS polymer on the market is produced by conventional emulsion polymerization techniques. Unfortunately, the prolonged polymerization periods and multi-step post polymerization processing necessary in emulsion type processes often render products made by this type of process economically unsuitable for many producers. In addition, residual emulsifier and other agents still present in the polymer due to incomplete purfication cause some deterioration in the properties of the polymer. For these reasons more efficient and economical processes for producing ABS polymers having acceptable properties are constantly being sought.
An additional problem heretofore presented by conventional ABS manufacturing processes is that the high levels of rubber (polybutadiene) which must be employed to obtain products having acceptable impact properties cause detrioration in other properties of the ABS such as surface properties, gloss molding properties and also present various reaction difficulties such as increased viscosity of the reaction mixture.
Recently, a patent has issued to DeBell et al (U.S. Pat. No. 3,370,105) which discloses a process wherein the rubbery material onto which the styrene and acrylonitrile are grafted is orginally in the form of a latex but is coagulated prior to the initiation of the polymerization. While this process presents certain advantages it still suffers from process disadvantages such as the need to add coagulant and results in a product which does not have the superior properties exhibited by the product of the present invention.
In another recently issued U.S. Pat. No. 3,436,440, Abe et al disclose an emulsion-suspension process involving the graft polymerization of a mixture of styrene and acrylonitrile onto a butadiene rubbery polymer in latex form. In this process the rubber latex, monomers, a suspending agent, a water-soluble catalyst and an oil-soluble catalyst are all introduced into a reactor; the contents are heated to polymerization temperature and the reaction proceeds in the emulsion state. After the polymerization has proceeded to a certain point the emulsion coagulates and forms a stable suspension. The polymerization is then completed in suspension. The process of this reference obviates many of the disadvantages associated with conventional emulsion procedures but the products prepared do not have the exceptionally good combination of properties exhibited by the products of the present invention. Furthermore, this reference does not disclose or suggest the process of the present invention wherein products having consistently good properties are prepared and wherein compositions of low rubber level and excellent properties are prepared.
There is therefore a need for a process for preparing ABS polymer which avoids the disadvantages of the prior art emulsion processes, which results in polymer having a desirable combination of properties such as light color, high impact, the flow, and which employs low rubber levels to prepare polymer having desirable properties.